There are various types of cartilage, e.g., hyaline, elastic and fibrocartilage. Hyaline cartilage is found at the articular surfaces of bones, e.g., in the joints, and is responsible for providing the smooth gliding motion characteristic of moveable joints. Articular cartilage is firmly attached to the underlying bones and measures typically less than 5 mm in thickness in human joints, with considerable variation depending on the joint and the site within the joint.
Adult cartilage has a limited ability of repair; thus, damage to cartilage produced by disease, such as rheumatoid and/or osteoarthritis, or trauma, can lead to serious physical deformity and debilitation. Furthermore, as human articular cartilage ages, its tensile properties change and the cartilage tends to wear away. The superficial zone of the knee articular cartilage exhibits an increase in tensile strength up to the third decade of life, after which it decreases markedly with age as detectable damage to type II collagen occurs at the articular surface. The deep zone cartilage also exhibits a progressive decrease in tensile strength with increasing age, although collagen content does not appear to decrease. These observations indicate that there are changes in mechanical and, hence, structural organization of cartilage with aging that, if sufficiently developed, can predispose cartilage to traumatic damage.
In osteoarthritis, human joints such as the knee, hip, ankle, foot joints, shoulder, elbow, wrist, hand joints, and spinal joints tend to wear away the articular cartilage. The wear is frequently not uniform, but localized to a defined region within a joint. For example, in a knee joint, the wear can be on a medial or lateral femoral condyle, a medial or lateral tibial plateau, a medial or lateral trochlea, a medial facet, lateral facet or median ridge of the patella. In a hip joint, the wear can be on an acetabulum or a femoral head or both.
Generally, wear can be limited to one articular surface or it can affect multiple articular surfaces. Wear can occur in one or more subregions on the same articular surface or multiple articular surfaces.
In a medial femoral condyle or medial tibial plateau, wear can occur in an anterior, central or posterior portion of the articular surface. Wear can also occur in a medial or lateral portion of an articular surface. In a lateral femoral condyle or lateral tibial plateau, wear can occur in an anterior, central or posterior portion of the articular surface. Wear can also occur in a medial or lateral portion of an articular surface. In a femoral head or acetabulum, wear can occur in an anterior, posterior, medial or lateral, superior or inferior location.
In short, any location of wear is possible, and any combination of wear patterns on the same articular surface and opposing articular surfaces is possible.
Wear starts typically in the articular cartilage, but it can then extend into the subchondral bone and marrow cavity. Wear can be accompanied by cartilage loss, subchondral sclerosis, subchondral cyst formation, osteophyte formation, bone marrow edema.
Wear is frequently the result of an abnormal biomechanical loading condition in a joint. While modern arthroplasty surgery attempts to correct such abnormal biomechanical loading conditions in a joint, some residual biomechanical loading abnormality or new biomechanical loading abnormality is frequently present after partial or total joint replacement surgery.
Abnormal biomechanical loading of joint implants is a frequent cause for implant failure since current implants cannot account for the increased loads and stresses resulting from such abnormal loading.
Usually, severe damage or cartilage loss is treated by replacement of the joint with a suitable prosthetic material, most frequently metal alloys. See, e.g., U.S. Pat. Nos. 6,383,228; 6,203,576; and 6,126,690. As can be appreciated, joint arthroplasties are highly invasive and require surgical resection of the entire (or a majority of) the articular surface of one or more bones involved in the repair. Typically with these procedures, the marrow space is fairly extensively reamed in order to fit the stem of the prosthesis within the bone. Reaming results in a loss of the patient's bone stock, and over time subsequent osteolysis will frequently lead to loosening of the prosthesis. Further, the area where the implant and the bone mate degrades over time requiring the prosthesis to eventually be replaced. Since the patient's bone stock is limited, the number of possible replacement surgeries is also limited for joint arthroplasty. In short, over the course of 15 to 20 years, and in some cases even shorter time periods, the patient can run out of therapeutic options ultimately resulting in a painful, nonfunctional joint.